This invention relates to the separation of suspended particles from a fluid by the application of a magnetic field. The invention involves the magnetic separation of ferromagnetic and paramagnetic particles from fluids by exposing a suspension of particles in a fluid to a magnetic field to cause the migration of particles under the influence of the field (due to the field gradient) thereby permitting recovery of a fluid product having a reduced solids concentration. Of recent interest is the technique known as high-gradient magnetic separation (HGMS). HGMS involves the interaction between a filtration element comprised of a ferromagnetic material such as wire elements and small ferromagnetic or paramagnetic particles in an applied magnetic field, i.e., a magnetic field provided by a source external to the ferromagnetic element. Magnetic field gradients around the elements are several orders of magnitude higher than in the absence of the ferromagnetic filtration element. The fluid feed stream containing suspended particles is passed in the vicinity of the ferromagnetic element. Those magnetic particles which pass within the capturing distance that the element presents to the fluid stream are caused to migrate to the element and are removed from the stream. In commercial practice the ferromagnetic element is in the form of a steel mesh and the external magnetic field is generally applied by an electromagnet. Superconducting electromagnets and permanent magnets have also been proposed for this application. An example of an HGMS system suitable for use according to this invention is described in the article "New Tasks For Magnetism", Chemical Engineering, January 7, 1974, pp. 50-52 which is incorporated herein by reference.
The applicability of magnetic separation techniques for removal of solids is dependent on a number of complex phenomena. For example, when magnetic separation is applied to a flowing fluid, the magnetic force must overcome fluid drag and in some cases gravitational forces, which are related to the size and density of the particles relative to the amount of magnetic material present. The recovery of magnetic particles from a stationary magnetic separator typically involves turning off the magnetic field and backflushing the filter media.